Numerous processes are already known for the preparation of arylamine/formaldehyde condensates, particularly of aniline/formaldehyde condensates, by reaction in the presence of aqueous acid catalysts and processing of the condensation mixture by distillation after neutralization of the catalyst and separation of the resulting organic phase. Polyamines based on diphenylmethane which are obtainable by these processes are primarily used as starting materials for preparation of the corresponding polyisocyanates.
In polyurethane chemistry, it is not only those polyisocyanate mixtures based on diphenylmethane series which are distinguished by a high 4,4'-diisocyanatodiphenylmethane content and low o-isomer content that constitute particularly valuable starting materials. For many fields of application, particularly where differing reactivities of the isocyanate groups are desired, there is special interest in polyisocyanate mixtures of the diphenylmethane series which contain a high proportion of 2,4'-diisocyanatodiphenylmethane. The proportion of o-isomers in polyamine mixtures is primarily a function of the strength and concentration of the acid catalyst used. If the aniline/formaldehyde condensation reaction is carried out in at least two stages, a low degree of protonation (degree of protonation=percentage of total quantity of nitrogen atoms present as ammonium groups) before the final condensation stage leads to an increased proportion of o-isomers in the end product. One disadvantage of this, however, is that in the processes known in the art, the often desirable increase in the concentration of 2,4'-isomers in the end products is invariably accompanied by an undesirable increase in the concentration of 2,2'-isomers.